250 
tensity in consequence of the unobstructed radiance through the 
fluting at the points of each carbon ; and (2) a steadier light is 
obtained owing to the localising of the current at the central por- 
tion of each carbon. 
The result of many experimental trials with fluted carbons 
50mm. diameter have entirely confirmed my expectations. No 
crater is formed in either of the carbon points, and their form is 
all that can be desired for utilising fully the maximum light of 
the radiant arc. My experiments have not been sufficient to de- 
termine accurately the additional intensity of light obtained from 
the arc of a pair of the fluted carbons as compared with that 
from the arc of a pair of cylindrical carbons, but I am of 
opinion that the gain with fluted carbons is not less than ten 
per cent. 
Geological Society, June 9.—Prof. J. W. Judd, F.R.S., 
President, in the chair.—The following communications were 
read :—On the volcanic rocks of North-Eastern Fife, by James 
Durham, F.G.S., with an appendix by the President. After 
describing the general distribution of the volcanic rocks of Old 
Red Sandstone and Carboniferous age in the counties of Forfar 
and Fife, the author called attention to a fine section exhibited 
where the Ochil Hills terminate along the southern shore of the 
Firth of Tay. In immediate proximity to the Tay Bridge, a 
series of the later volcanic rocks, consisting of felstones, breccias, 
and ashy sandstones are found let down by faults in the midst of 
the older porphyrites (altered andesites) which cover so large an 
area in the district. The breccias contain enormous numbers of 
blocks of a red dacite (quartz-andesite), and inclosed in this 
rock angular fragments of a glassy rock, resembling a ‘‘ pitch- 
stone-porphyry,” are found, everywhere, however, more or less 
converted into a white decomposition-product. The youngest 
igneous rocks of the district are the bosses and dykes of melaphyre 
(altered basalt and dolerite) which have been often so far re- 
moved by weathering as to leave open fissures. 
three very interesting rocks were described in detail. The rock 
of the Northfield Quarry, which is shown to be the augite- 
andesite, has a large quantity of a glassy base with felted micro- 
lites, and contains large porphyritic crystals of a colourless 
augite. The rock of the Causewayhead Quarries is described 
as an enstatite-andesite ; it has but little glassy base, being made 
NATURE 
In the appendix | 
[%uly 1, 1886 
apparent justification for the mapping of both of these rocks as 
“ec 
greenstone ” by the Geological Survey.—The Bagshot beds of 
the London Basin, by H. W. Monckton, F.G.S., and R. S. 
Herries, B.A., F.G.S. The authors stated that their object was 
to describe more fully the Lower Bagshot beds, and to disprove 
the view lately advanced by Mr. Irving that, in certain places, © 
the Upper Bagshots overlap the Lower, and rest directly on the 
London Clay. They described or referred to a number of sec- 
tions all round the main mass, beginning at St. Ann’s Hill, 
Chertsey, where they considered that the mass of pebbles and 
associated greensands must be referred to the Middle Bagshot. 
The outliers near Bracknell and Wokingham were shown to 
consist of Lower and Middle Bagshot, which does not appear 
in the valley north of Wellington College. The Aldershot dis- 
trict was explained, and it was shown that the beds there resting 
on the London Clay were Lower and not Middle Bagshot, and 
the occurrence of fossils in the Upper Bagshot of that district 
was recorded. The conclusions that the authors came to were, 
that a well-marked pebble-bed was almost always present, mark- 
ing the division between the Upper and Middle Bagshots, but 
that there were other pebble-beds of a less persistent character 
occurring both in the Middle and Lower Bagshots ; that the 
Lower Bagshots generally consist of false-bedded sands with 
clay laminze and no fossils except wood, whereas the Upper Bag- 
shots are rarely false-bedded, and are characterised by the absence 
of clay bands and the presence of marine fossils; and that 
the Middle Bagshot is a well-marked series consisting of 
greensands and clays. They claimed, in conclusion, that there 
was no reason for disturbing the old reading of the district, and 
that there was no evidence of an overlap of the Lower Bagshots 
by the Upper. 
Physical Society, June 12.—Dr. J. H. Gladstone, Vice- 
President, in the chair.—Dr. Samuel Rideal and Mr. E. C. 
Wellington were elected Members of the Society.—The follow- 
ing communications were read :—On an electric-light fire-damp 
indicator, by Messrs. Walter Emmott and William Ackroyd. 
| The Royal Commission on Accidents in Mines point out in their 
up of lath-shaped felspar crystals (andesine), with prismatic | 
crystals and grains of a slightly ferriferous enstatite ; there are 
no porphyritic crystals, but the enstatite individuals are some- | 
times curiously aggregated. The red porphyritic rock from the 
breccias near the Tay Bridge was shown to be a mica-dacite, 
and the glassy rock associated with it to be the same material 
with a vitreous in place of a stony base. The glassy base ex- 
hibits very beautiful fluidal and perlitic structures. The crystals 
of first consolidation in this rock are oligoclase and biotite, often 
showing marks of injury in transport ; those of the second con- 
solidation appear to be orthoclase. In conclusion, the successive | 
stages by which the andesitic rocks of the area were altered, so 
as to assume the characters distinctive of porphyrites, were fully 
discussed, as well as the change of the glassy rock into its 
white decomposition-product.—On some eruptive rocks from 
the neighbourhood of St. Minver, Cornwall, by Frank Rutley, 
F.G.S. The rocks described in this paper were derived from 
Cant Hill, opposite Padstow, and from a small quarry about 
half a mile from Cant Hill, near Carlion. At the former locality 
the volcanic rocks are much decomposed, but from their micro- 
scopic characters they may be regarded as altered glassy lavas 
of a more or less basic type. No unaltered pyroxene, amphi- 
bole, or olivine is to be detected in the specimens described, 
but there is a considerable amount of secondary matter which 
may include kaolin, serpentine, chlorite, palagonitic substances, 
&c. There is evidence of fluxion-structure in some of the sec- 
tions ; others are vesicular, and the vesicles are usually filled 
with siliceous or serpentinous matter. The relation of these 
lavas to the underlying Devonian slates was not ascertained. 
The rock occurring near Carlion contains numerous porphyritic 
crystals of augite in which the crystallisation is interrupted by 
the co-development of small felspar crystals, which appear, as a 
rule, to have been converted into felsitic matter. Ilmenite is 
also present in patches which indicate a similar interrupted 
crystallisation to that shown by the augite. The rock has the 
mineral constitution of an augite-andesite, but since it is a holo- 
crystalline rock, exception would be taken by many petrologists 
to the employment of the term andesite. The lavas of Cant 
Hill were also probably of an andesitic character, so that, so 
far as original mineral constitution is concerned, there is some 
recently-issued report, as a serious objection to the use of the 
electric light in mines, notwithstanding its many great ad- 
vantages, that the light of an incandescent lamp, being pro- 
duced within a vacuum, cannot admit of any device for the indi- | 
cation of fire-damp such as is.given by the Davy, for example. 
The present apparatus is the outcome of an attempt to overcome 
this difficulty. It consists of two incandescent lamps, one with 
colourless and the other with red glass, and the circuit is so 
arranged that in an ordinary atmosphere the colourless lamp 
alone shines, but in fire-damp this goes out, and the red one is 
illuminated. This is effected in a simple manner by the motion 
of a mercury contact occupying the lower part of a curved tube, 
one end of which is open, and the other connected witha porous 
pot of unglazed porcelain, the motion of the mercury being due 
to the increased pressure in the porous pot occasioned by diffu- 
sion. —On a method of distinguishing rays of solar from those of 
terrestrial origin, by Prof. Cornu. It has been shown by M. 
Fizeau that, owing to the rotation of the sun upon its axis, there 
is a disp]acement of the spectral lines produced by solar absorp- 
tion towards the red or towards the violet, according as to 
whether the light examined emanates from those parts of the 
sun which are receding from or approaching us. If, however, 
the lines are the result of absorption by the earth’s atmosphere 
no such displacement should occur. It has been the aim of the 
author to make this principle the basis of a simple and instan- 
taneous method of determining the origin of any given line. The 
displacement is very minute, amounting to about 1/150 of the 
distance between the D lines for rays in that part of the spec- 
trum when the light is from the extremity of the solar equator, 
but it has been found quite sufficient. Observations have been 
made with a Rowland grating, the mean distance of the lines 
being ‘00176 mm. An image of the sun is formed upon the slit 
of the spectroscope by a lens. By a slight oscillatory motion 
given to the lens by a Jever from the hand, any part of the sun’s 
image can be brought upon the slit. A heliostat sends the rays 
always in the same direction, and by a prism the image has its 
equator horizontal. To distinguish between a line of solar and 
one of terrestrial origin the line is brought near the vertical wire 
of the eye-piece, or, better still, one of those inevitable grains of 
dust which are always seen on the horizontal wire. The lever 
connected to the lens is then oscillated so as to bring alternately 
the two ends of the solar equator tangentially upon the 
slit. If the ray is of terrestrial origin it remains abso- 
“es eee ee 
ee eneinens Sees > 
